1. Field of the Invention
The present invention relates to a process for reducing attendant color shift in coatings containing blocked isocyanates. More specifically, the present invention relates to reduction in yellowing of the coatings and its effect on topcoats.
2. Brief Description of the Prior Art
Curable compositions containing blocked isocyanate crosslinking agents are known in the art. In instances of aromatic-containing blocked isocyanates, one can encounter an associated problem of yellowing topcoats or the like which are in contact therewith. Yet, another associated problem with aromatic-containing blocked isocyanates is the yellowing of topcoats or the like, in contact therewith. In many instances the above-associated problems are solved at the expense of other desirable coating properties such as stability, throw power, film build, cure temperature, and corrosion resistance.
Hence, work in this area has continued in developing blocked isocyanate curing agents which would avoid the associated problems without compromising other desirable coating properties. The work done has shown that proper selection of isocyanates and blocking agents for the isocyanate groups is determinative of the performance of the curable compositions containing the same.
One is, therefore, faced with a myriad of combinations of isocyanate and blocking agents that would produce the appropriate blocked isocyanates which would overcome the afore-stated problems. By the present invention, a specific combination of an isocyanate and blocking agent has been found effective in overcoming the afore-stated problems.
To be sure, in the art of preparing blocked isocyanates, there has been disclosed enumerable isocyanates which are said to be reactable with enumerable active hydrogen containing materials. There is, however, no basis in the prior art for selecting a specific isocyanate and a blocking agent in accordance with this invention, in order to obtain the remarkable combination of coating properties described hereinbelow.